Infinite resistance of a Voltmeter

While studying about 'Potentiometer' it's said :
"The resistance of the voltmeter be large compared to the circuit resistance across which the voltmeter is connected.Otherwise an appreciable current will flow through voltmeter which will alter the circuit current & the P.D to be measured". Can any body explain this concept?.Thanks.

The concept is actually straight forward. You need to show some thought about your question before we can help you. (see post)

If you are truly lost, you might begin by defining some of the terms or if it is a measuring device, how do you normally use this in the lab. Have you done any research about this, on the web? (hint: use search engines)..

Think of two circuits each with a battery and a two equal resistors of say 1kohm, one where you are measuring the voltage across a resistor with a high resistance voltmeter say 1Mohm and one where you measure with a low resistance voltmeter say 1ohm. Draw out the circuits and see if you can work out what the voltage measured in each case would be and what the actual voltage should be.

While studying about 'Potentiometer' it's said :
"The resistance of the voltmeter be large compared to the circuit resistance across which the voltmeter is connected.Otherwise an appreciable current will flow through voltmeter which will alter the circuit current & the P.D to be measured". Can any body explain this concept?.Thanks.

Because "an observer disturbs the system he/she is trying to observe", it is important to include a measuring device in a such way that only minimal change to the original system results. In the voltmeter case, you are providing the current an alternative pathway to flow once you have introduced the voltmeter in the circuit. Hence, in order to minimise the amount of currrent using this path (created by the inclusion of the voltmeter) you want the resistance of the voltmeter to be very large compared to the circuit component it is measuring. Higher resistance means that most of the current will follow the original path and hence leads to minimal change to the circuit conditions.

I know both of you, are trying to be helpful to shehri, and you have not earned "homework helper" badges yet. It would be more helpful to the person asking the question, to show they have given some thought to them and not just seeking answers from the PF community.

You are encouraged to read this sticky note which outlines what is expected of posters to the homework section.